Coreless induction furnaces for melting or otherwise heating metal by inducing eddy currents to flow within the metal are well known. (As used in this disclosure, the term "heating" is used broadly to encompass not only raising the temperature of a material without causing the material to change state, but also melting, wherein the temperature of a material is raised sufficiently to cause it to change state.) Typically, metal to be heated is contained in a vessel, usually but not always a crucible. Eddy currents are electromagnetically induced in the metal by an induction coil surrounding the vessel. The eddy currents cause power to be dissipated in the metal, thereby increasing its temperature. In effect, the metal serves as its own heat source. The eddy currents are induced in the metal when alternating current is passed through the induction coil to generate an alternating magnetic field, or induction field. Depending upon the frequency of the alternating current in the induction coil, and on other design considerations, an induction furnace may be used for either heating or physically agitating ("stirring") a quantity of molten metal, and sometimes both.
The heating vessel must meet certain stringent physical standards. It must have a sufficiently high melting point so that it will not be melted by the heat of the metal, it must have a high strength to hold the weight of the metal, and it must not interfere with the passage of magnetic flux from the induction coil through and around the metal. Very often, these requirements of high melting point, strength, and lack of interference with the applied magnetic induction field are at cross-purposes from a design standpoint.
The present invention optimizes the competing design criteria, enables induction heating vessels to be made larger and stronger than previous designs, and offers dramatic improvements in efficiency. The present invention is particularly-well suited for removable heating vessels for coreless induction furnaces. Removable crucible induction furnaces have been known since at least the early part of this century. See, e.g., Helberger U.S. Pat. No. 1,023,309, which discloses a ceramic refractory crucible which is partially sintered in place and which is pushed out of the induction coil by a hydraulic ram. In practical terms, however, the utility of a crucible such as that disclosed by Helberger is limited by the use of ceramic alone for the crucible. Ceramic, as is well known, is brittle and subject to stress cracking. Breakage of the ceramic crucible could lead to "run out" of molten metal from the crucible, posing a severe safety hazard to operating personnel and to equipment, as well as leading to loss of the metal charge. Thus, ceramic crucibles tend to be relatively small and limited to small quantities of metal.
One way of strengthening a ceramic crucible is to surround it by a continuous metallic jacket, or shell. Metals are generally less brittle and have higher yield strengths than ceramics. The shell material is typically steel. However, since steel and many other useful metals are either electrically conductive, magnetic, or greatly weakened when heated, steel-jacketed ceramic crucibles do not offer much of an improvement over ceramic crucibles alone since the magnetic field generated by the induction coil will heat the shell as well as the molten metal in the crucible, thus rendering the vessel either useless or unsafe.
Where metal-jacketed ceramic crucibles have been used in induction furnaces, their use has generally been restricted to induction stirring, not induction heating. For example, Kennedy U.S. Pat. No. 3,314,670 describes a vessel having a one-piece, electrically continuous outer shell of austenitic stainless steel with a refractory lining. The steel must be specially selected to have very specific electrical and magnetic properties. The Kennedy vessel is capable of use only within a frequency range of 0.1 Hz to 60 hz (column 2, line 59). Further, the Kennedy vessel is limited to stirring, and the Kennedy patent states that stirring may take place effectively only within a narrow range of parameters, taking into account the competing requirements of temperature range, stirring forces, strength, and economics. Kennedy thus recognizes that his vessel is not usable for induction melting.
Other attempts at developing usable metal-jacketed ceramic crucibles involve the selection of exotic materials. For example, Willay U.S. Pat. No. 4,446,563 discloses a composite vessel with an inner container made of vitreous carbon and an outer container made of platinum. Clearly, however, such exotic designs are extremely impractical and are not cost effective for most applications.
There thus exists a definite need for a metal-jacketed induction heating vessel that overcomes the drawbacks of the prior art. The present invention provides such a vessel by providing a method by which a metallic shell can be rendered substantially transparent to an applied induction field and by which eddy currents induced in the shell are greatly reduced, thus reducing heating in the shell. A metal-jacketed induction heating vessel employing the principles of the present invention does not rely on exotic materials, nor is it limited to only inductive stirring of molten metals. A metal-jacketed induction heating vessel according to the present invention is, in contrast to prior vessels, able to handle large quantities of metal, with excellent yield strengths at operating temperatures. At the same time, a vessel according to the present invention allows most of the electromagnetic field developed by the induction coil to couple with the metal to be melted, with only a small amount coupling to the metal jacket. This also is a substantial improvement over prior metal-jacketed crucibles, in which a great deal of energy was lost in coupling between the induction coil and the metal jacket itself.
Moreover, the present invention is not limited to ceramic-lined vessels for coreless induction furnaces. The present invention includes metal-jacketed induction heating vessels which have an inductively heatable susceptor, such as a graphite susceptor, in which non-conductive materials may be heated indirectly by inductively heating the susceptor.
These and other advantages of the present invention will become apparent from the following description.